Artificial dielectric polarizer



ll INDEX OF REFRACTION Jan. 12, 1960 A. F. WICKERSHAM, JR

ARTIFICIAL DIELECTRIC POLARIZER EXAMWER f- FREQUENCY lNVENTOR ARTHUR F. WICKERSHAM,JR.

ATTORNEY United States Patent ARTIFICIAL DIELECTRIC POLARIZER Arthur F. Wickersham, In, Santa Clara, Calif., assiguor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Application December 26, 1957, Serial No. 705,191

16 Claims. (Cl. 343-911) This invention relates to artificial dielectrics and, in particular, to an artificial dielectric polarizer exhibiting a substantially constant phase shift over a broad band of frequencies.

The term artificial dielectric as used herein is understood to mean obstacle-type, wave-propagating media comprising fabricated assemblies of conducting or nonconducting members or both. Such dielectrics are used principally in microwave lens antennas where their advantages over natural dielectrics include lightness, mechanical strength and stability, and a choice of any desired index of refraction over wide limits. Other devices in which artificial dielectrics find application include polarizers and matching walls.

To have utility in a quarterwave plate or polarizer, a dielectric medium must satisfy the condition where A is the optical path difference between two rays traversing the medium, and where the first ray is polarized in a principal plane and the second ray is polarized in a plane perpendicular to the first. The index of refraction, 1;, in the medium is approximately related to the optical path difference A by the expression where D is the thickness of the dielectric medium. Inasmuch as )t decreases with increasing frequency, and 1; (or A) in natural dielectrics increases with increasing frequencies, it is clear that the condition expressed in Equation 1 can be met with natural dielectrics at only one frequency.

It is a primary object of the present invention to provide a quarterwave plate or polarizer which operates over a broad band of microwave frequencies.

Another object of the invention is to provide an artificial dielectric which has a dispersion curve which decreases with increasing frequency over a wide frequency range so as to satisfy Equation 1. A dis ersion curve as used herein, means a plot of the index of refraction of the dielectric against freguency.

- Still another object of the invention is to provide an artificial dielectric wherein the optical path difference between two orthogonally polarized waves is a decreasing function of frequency so as to be operative as a quarterwave plate or polarizer over a wide frequency range.

The foregoing objects are achieved by the formation of a compound artificial dielectric consisting of arrays of conducting scattering elements arranged in a supporting dielectric material. More specifically, the compound artificial dielectric comprises a plurality of stacked thin sheets of low density dielectric material, such as Polyfoam (polystyrene foam) on each of which is supported an array of thin narrow conducting rectangles, the longitudinal axes of the rectangles being orthogonally disposed on alternate supporting layers. Thus alternate sheets have principal planes which are mutually orthogoma], and the arrays are arranged whereby alternate layers have different regions of resonance; that is, the conducting elements of these arrays on the odd numbered sheets may be arranged to be resonant at a frequency near the lower end of the frequency range over which it is desired that the polarizer shall be operative, and the arrays supported by even numbered sheets may be arranged to be resonant at a frequency near the upper end of the desired operating range.

The pass bands of the alternate sheets being relatively broad between resonance points, the compound dielectric may be made to exhibit an optical path difference between orthognal polarizations which decreases with frequency in such a manner as to satisfy Equation 1 over a wide frequency range.

The nature of the invention will be better understood from the following detailed description of an illustrative embodiment thereof, reference being had to the accompanying drawings in which:

Figure l is an exploded view of the compound artificial dielectric made in accordance with the present invention;

Figure 2 is a perspective view, partially cut away, of the compound artificial dielectric; and

Figure 3 is a graph illustrating the dispersion characteristics of the components of the compound dielectric.

Referring to the drawings, a compound artificial dielectric made according to the present invention comprises a plurality of thin layers or sheets 11, 12, 13, 14, 15 and 16 of light-weight dielectric material, such as Polyfoam, stacked together to form the dielectric assembly 18 shown in Figure 2. The dielectric sheets support planar arrays of scattering elements 20 and 21 consisting of thin, elongated, retangular strips of conducting material, such as metallic foil or silver paint, and the strips are arranged on each sheet with their longitudinal axes parallel. The elements on each sheet are uniformly spaced apart both longitudinally and laterally, and-form'"several parallel rows which make up the square array shown in the drawings. The dielectric sheets may be fabricated, for example, by silk screening stripsof silver paint on a face of each shfiet andthen bonding the several sheets together in a stack with the strips on adjacent sheets separated by the thickness of one layer.

In order that the dielectric assembly 18 shall exhibit the desired dispersion curve, the arrays on alternate sheets in the stack, say sheets 11, 13 and 15 are made in such a manner as to have a dispersion curve which combines with the dispersion curve of the other arrays on sheets 12, 14 and 16 to produce a net result that satisfies the aforementioned conditions for a quarterwave plate. The whole dielectric assembly 18 therefore is made up of two dielectric subassemblies each of which is designed to have a dispersion curve of predetermined characteristics. The arrays on the odd numbered sheets, for example, comprise a first dielectric while the second dielectric consists of the arrays on the even numbered sheets. By properly selecting the physical and geometrical parameters of the scattering arrays for these two dielectric subassemblies, the pass bands between resonant frequencies of each can be made relatively broad, and the particular frequency at which each dielectric is resonant may be selected independently.

The compound dielectric 18 is assembled by stacking the sheets of the two dielectric subassemblies in alternating sequence so that a sheet of one is adjacent to a sheet of the other. The arrays of one dielectric subassembly are identical, and similarly the arrays which comprise the second dielectric subassembly are alike. However the principal planes and the regions of resonance of the two dielectrics are difierent. In other words, one dielectric subassembly exhibits its dielectric properties in a plane which is orthogonal to the principal plane of the other, and

the two subasscmblies are resonant at widely spaced frequencies. The principal plane of each array is parallel to the long axes of the scattering elements in that array, these elements corresponding essentially to an array of well known dipoles. According to the invention, the scattering elements in one set of arrays are oriented with their long axes displaced 90 degrees from the axes of elements in the other set of arrays. Thus the principal planes of arrays on adjacent sheets are mutually orthogonal.

In order that the arrays of conducting elements in one constituent dielectric shall be resonant at one frequency, for example, at F in Figure 3, and the arrays on the other dielectric shall resonate at a substantially higher frequency, such as at F;, the arrays on one sheet are made to differ physically from the arrays on the adjacent sheet. This may be effected in a number of ways, for example, by varying the number, intermediate spacing, shapes, conductivities, and/or thicknesses of the strips. In other words, the distribution" of the strips in one set of arrays is different from the distribution" of the strips in the other arrays, where the term distribution" as used herein and in the claims means any physical difference, including differences in configuration and/or constitution, between the two types of strips which cause them to resonate at different frequencies. In one form of our invention, elements 21 are made longer than elements to achieve the aforementioned purpose. Also, the lateral spacing between adjacent elements on the same array has a pronounced effect on the resonant characteristics, and variation in such spacings in adjacent arrays provides desired control of resonance points. Through selection of such physical parameters of the scattering elements, the arrays on the odd numbered sheets may be made to resonate at a frequency near one end of the desired frequency range, and the arrays on the even numbered sheets likewise may be made to resonate at a frequency near the upper end of the frequency range.

The dispersion curves for the two constituent dielectrics are shown in Figure 3 wherein curve 24 represents the dispersion curve for the dielectric comprising the odd numbered sheets and curve 25 represents the dispersion effects of arrays on the even numbered sheets. It will be noted that curve 24 is resonant at frequency F and curve 25 is resonant at a substantially higher frequency F Between these resonant frequencies, the two dispersion curves have positive slopes with the slope of curve 25 being smaller than that of curve 24. By proper choice of parameters for the arrays of the two dielectrics, the slopes of the curve as well as the resonant points can be selected. A preferred frequency range for normal operation with the compound dielectric is in the second pass band region of the one constituent dielectric and in the first pass band region of the other dielectric, that is, the frequency range that lies above resonant frequency F for the one dielectric and below resonant frequency F for the second dielectric. In this region, indicated by double arrows in Figure 3, the slope of curve 25 is somewhat flatter than that of curve 24. The optical path difference between orthogonal polarization components of radiation passing through the compound dielectric is proportional to the difference between the two curves which, it will be noted, decreases with increasing frequency. Thus the condition in which the optical path difference A decreases in such a manner as to satisy Equation 1 over a wide frequency range is met.

The compound dielectric 18, to serve as a quarterwave plate, is oriented so that the incident radiation of microwave energy propagates through it normal to the plane of the sheets. This radiation should be a linearly polarized wave whose E-vector, see Figure 2, is at an angle of 45 with the axes of the elements comprising the dielectric arrays, so that the E and E components are either parallel to or perpendicular to the axes of the scattering elements. As the B component of the wave passes through the arrays on sheets '12, 14 and 16, there is a negligible efiect because of the perpendicular relationship, and the E component is undisturbed. This component, however, is parallel to axes of elements 20 on the odd numbered sheets and therefore is responsive to these elements for its phase velocity. The component 15,, on the other hand is affected by array elements 21 on the even number sheets but is not affected by elements 20. Thus the phase of the two components passing through the medium can be relatively controlled so that the optical path difference of the two components in the compound dielectric of thickness D is a decreasing function of frequency over a wide band of operating frequencies, and the phase shift caused by compound dielectric is substantially constant.

By way of example, the following are dimensions, physical data and performance characteristics of particular components suitable for fabricating a broadband quarterwave plate in accordance with the present invention:

From the foregoing description, it will be noted I have provided an anisotropic artificial dielectric which exhibits a difference in the two principal refractivities that decreases with frequency over a wide frequency range and therefore meets the conditions of a quarterwave polarizer. The dielectric is readily constructed from light-weight mechanically stable material which can be economically produced.

While a preferred embodiment of this invention has been described in detail, it will be appreciated by those skilled in the art that various changes can be made in the detailed construction and the arrangement in parts without departing from the principles of the invention and the scope of the appended claims.

What is claimed is:

1. A compound artificial dielectric comprising a plurality of stacked dielectric sheets, each of said sheets supporting a plurality of thin elongated conducting strips, said strips being spaced apart on each sheet and having their longitudinal axes parallel, the strips on adjacent sheets having different resonant frequencies and being arranged with their axes orthogonally related.

2. An artificial dielectric comprising a plurality of arrays of elongated conducting elements, said arrays being stacked together, the elements of each array being sup ported in spaced relation in a common plane and being oriented with their long axes parallel with and orthogonal to the axes of the elements of the adjacent arrays in the stack.

3. A compound artificial dielectric through which microwave energy is propagated, comprising a first constituent dielectric and a second constituent dielectric, each of said first and second dielectrics comprising a plurality of arrays of scattering elements stacked along an axis with the successive arrays disposed in parallel planes normal to the direction of microwave propagation, said scattering elements in each array having principal planes of resonance orthogonally related to the planes of resonance of the elements in the adjacent array, the distribution of said elements in adjacent arrays being difierent such that the resonant frequencies of said first and second dielectrics are widely spaced.

4. The compound dielectric according to claim 3 in which the lengths of the elements of one array are equal and are different from the lengths of elements in an adjacent array.

5. The compound dielectric according to claim 3 in which the lateral spacings between elements on one array are equal and are different from the lateral spacings between elements on the adjacent array.

6. A compound artificial dielectric through which electromagnetic waves are propagated, comprising a first dielectric medium and a second dielectric medium stacked together in the direction of wave propagation, said first and second media having difierent principal planes of resonance and different resonant frequencies, said media also having indexes of retraction which change at different rates with increases in frequency of the propagated wave whereby the combined effect of the two media is a decreasing difference of refractive indexes with increasing frequency.

7. The dielectric according to claim 6 in which said first and second dielectric media each comprise a plurality of layers of scattering elements.

8. The dielectric according to claim 7 in which the layers which comprise the first medium are alternated with the layers of the second medium.

9. A microwave quarterwave plate adapted to transmit electromagnetic waves, comprising a first dielectric medium and a second dielectric medium, said first and second media having orthogonally related planes of resonance and having resonant frequencies at opposite ends, respectively, of a given frequency band, the rate of change of index of refraction of said first medium with change of frequency being greater than the rate of change of index refraction of the second medium with change of frequency whereby the difierence between refractive indexes of the two media decreases with increases in frequency.

10. A quarterwave plate according to claim 9 in which each dielectric medium comprises a plurality of planar arrays of scattering elements, said arrays of the first and second media being stacked together with the planes of the arrays normal to the direction of propagation of said electromagnetic waves.

11. A quarterwave plate according to claim 10 in which said arrays of one medium are alternated with the arrays of the other medium.

12. A compound artificial dielectric through which electromagnetic waves are propagated and having an axis along the direction of propagation, comprising a first constituent dielectric and a second constituent dielectric, said first dielectric having a plurality of thin parallel axially spaced non-conducting sheets, each of said sheets supporting a scattering array comprising a plurality of spaced elongated conducting strips oriented with their axes parallel and extending in a first direction, said second dielectric comprising a plurality of parallel axially spaced non-conducting sheets, each of which latter sheets supports an array of elongated spaced conducting strips have parallel axes extending in a second direction perpendicular to said first direction, said first and second dielectrics being stacked together such that sheets of each are normal to and symmetrical about said axis and are alternated with the other with adjacent arrays insulated from each other.

13. The compound artificial dielectric according to claim 12 in which the number of sheets in said first and second dielectrics is such that the optical path dilierence between two components of an electromagnetic wave traversing the sheets of said first and second dielectrics and polarized respectively in said first and second directions is a quarter of the wave length.

14. A quarterwave plate comprising a plurality of planar arrays of elongated metallic strips, said arrays being stacked along an axis with adjacent arrays axially spaced apart, the strips on adjacent arrays being mutually orthogonal and having different lengths.

15. A quarterwave plate according to claim 14 in which said strips are rectangularly shaped.

16. A quarterwave plate comprising a plurality of planar arrays of elongated metallic strips, said arrays being stacked along an axis with adjacent arrays axially spaced apart, the strips on adjacent arrays being mutually orthogonal and being differently laterally spaced.

No references cited. 

